The present invention relates generally to controlled current inverters and more particularly to a scheme for the recovery of an inverter bridge operation malfunction in which the commutating capacitor voltage is insufficient to effect proper controlled rectifier commutation.
It has been found when a controlled current inverter having a controlled rectifier bridge is employed to control an ac motor in the regenerative braking mode, the situation sometimes develops where the voltage on a commutating capacitor is not sufficient to effect the commutation (turning off) of one bridge controlled rectifier when the next controlled rectifier is rendered conductive. If this situation is permitted to continue, control of the bridge may be lost and the operation thereof may degrade until all control is lost and the bridge operates as a diode rectifier bridge. This problem has been found to be particularly prevelant in situations in which two bridges are used to control a single motor. An example of such dual bridge operation is found in co-pending U.S. Pat. No. 4,349,772 for "Method and Apparatus for Controlling An Alternating Current Motor Load Using Plural Controlled-Current Inverter Circuits" by Herbert W. Weiss, issued Sept. 14, 1982 which patent is assigned to assignee of the present invention. In this type of system, two six-pulse inverters are connected to transfer power between a motor and a source of power. When these inverters are of the controlled current type and the motor is operating in the regenerative mode such that the resultant power flow is from the motor to the source, it has been discovered that in certain instances only one of the six-pulse inverters is required to maintain motor flux and, thus, operate as a forced commutated inverter. Depending upon the circumstances, the other inverter may operate in varying degrees of load commutation or may, in fact, degrade to operation as a simple diode rectifier bridge such that the full capabilities of the equipment are not realized and totally reliable operation is not achieved.